1. Field of the Invention
The present invention relates generally to the replacement of a natural knee joint with a knee prosthesis and pertains, more specifically, to achieving better emulation of natural knee joint kinematics in a prosthetic knee.
During articulation of a natural knee joint, flexion between the tibia and the femur takes place about a transverse axis while, at the same time, some relative rotation between the tibia and the femur occurs about a longitudinal axis. Such flexion and rotation is necessary to carry out a normal gate cycle. It has been established that in full extension the tibia is rotationally displaced, relative to the femur, by approximately 2° to 3°. As the natural knee flexes, the tibia rotates internally. According to previous studies, about 5° of rotation ordinarily occurs as the knee is articulated from 0° to 10° of flexion; thereafter, little further rotation occurs up to at least about 45° of flexion. Total rotation at 110° of flexion is approximately 20°.
2. Description of the Related Prior Art
Rotational stability of the natural knee is provided by the collateral and cruciate ligaments. The cruciate ligaments deter uncontrolled internal rotation within a certain range of flexion of the knee, while the collateral ligaments provide transverse stability and deter uncontrolled external rotation of the tibia. Where the natural knee is replaced by a total knee prosthesis, either the anterior cruciate ligament or both the anterior and posterior cruciate ligaments ordinarily are sacrificed. In these instances, the knee prosthesis usually is provided with tibiofemoral articular constraint to supply the stability ordinarily provided by the sacrificed anterior cruciate ligament and a stabilizing mechanism for supplying the stability ordinarily provided by the sacrificed posterior cruciate ligament.